2.
Image Structure Related Dialogs

The following dialogs let you control and manipulate image structures,
such as layers,
channels, or
paths.

2.1. Layers Dialog

Figure 14.1. Layers Dialog

The “Layers” dialog is the main interface to edit, modify
and manage your layers. You can think of layers as a stack of slides or
clothes on your body. Using layers, you can construct an image of several
conceptual parts, each of which can be manipulated without affecting any
other part of the image. Layers are stacked on top of each other. The
bottom layer is the background of the image, and the components in the
foreground of the image come above it.

2.1.1. Activating the dialog

from the Tab menu in any dockable dialog by clicking on
and selecting
Add Tab → Layers,

from the (default) shortcut:
Ctrl+L.

In the Windows menu, there is a list of
detached windows which exists
only if at least one dialog remains open. In this case, you can raise
the “Layers” dialog from the image-menu:
Windows → Layers.

2.1.2. Using the Layer dialog

Overview

Every layer appears in the dialog in the form of a thumbnail. When
an image has multiple layers as components, they appear as a list.
The upper layer in the list is the first one visible, and the
lowest layer the last visible, the background. Above the list one
can find characteristics related individually to each layer. Under
the list one can find management buttons for the layer list. A
right-click in a layer thumbnail opens the Layer
context menu.

Layer attributes

Every layer is shown in the list along with its attributes:

Layer visibility

In front of the thumbnail is an icon showing an eye. By
clicking on the eye, you toggle whether the layer is visible
or not. (Shift-clicking on the eye causes all
other to be temporarily hidden.)

Chain layers

Another icon, showing a chain, allows you to group layers
for operations on more than one layer at a time (for example
with the Move tool).

Layer name

The main attribute is the name of the layer. You can edit
this by a double-click on the name or the thumbnail of the
layer.

Note

In the case of an animation layer (GIF or MNG), the name of the
layer can be used to specify certain parameters : Layer_name
(delay in ms) (combination mode), for example Frame-1 (100 ms)
(replace). The delay
sets the time during which the layer is visible in the
animation. The combination mode sets whether you combine the
layer with the previous layer or replace it: the two modes are
(combine) or (replace).

Layers characteristics

Above the layer list, it is possible to specify some properties
for the active layer. The active layer is the one highlighted in
blue. The properties are : "Layer mode", "Lock Alpha channel", and
"Opacity".

Layer modes

The layer mode determines how the layer interacts with the
other layers. From the combo box you can access all the
modes provided by GIMP. The layer modes are fully detailed
in Section 2, “
Layer Modes
”.

Lock

If you check this option the transparent areas of the layer
will be kept, even if you have checked the
Fill transparent areas
option for the Bucket fill tool.

Figure 14.3. Example for Locking Alpha Channel

The active layer has three horizontal, opaque, green
stripes on a transparent background. We paint a
vertical red stripe. “Lock” unchecked:
Opaque and transparent areas of the active layer are
painted with red.

“Lock” checked: Only opaque
areas of the active layer are painted with red.
Transparent areas are preserved.

Tip

If a layer name in the Layer Dialog is in bold, then this
layer has no Alpha channel.

Opacity

By moving the slider you give more or less opacity to the
layer. With a 0 opacity value, the layer is transparent and
completely invisible. Don't confuse this with a Layer Mask,
which sets the transparency pixel by pixel.

Layer management

Under the layer list a set of buttons allows you to perform some
basic operations on the layer list.

New layer

Here you can create a new layer. A dialog is opened where
you can enter the Layer name,
perhaps change the default Height
and Width, and choose the
Layer fill type
that will be the new layer's background.

Raise layer

Here you can move the layer up a level in the list. Press
the Shift
key to move the layer to the top of the list.

Lower layer

Here you can move the layer down a level in the list. Press
the Shift
key to move the layer to the bottom of the list.

Tip

To move a layer at the bottom of the list, it may first be
necessary to add a transparency channel (also called Alpha
channel) to the Background layer. To do this, right click
on the Background layer and select
Add Alpha channel from the menu.

Duplicate layer

Here you can create a copy of the active layer. Name of new
layer is suffixed with a number.

Anchor layer

When the active layer is a temporary layer (also called
floating selection) shown by this icon
,
this button anchors it to the previous active layer.

Delete layer

Here you can delete the active layer.

More layer functions

Other functions about layer size are
available in the Layer
Drop down menu you get by right clicking on the
Layer Dialog. You can find them also in the Layer sub-menu
of the image menu.

Click and hold on layer thumbnail: it enlarges and you can
move it by dragging the mouse.

So you can put this layer down
somewhere else in the layer list.

You can also
put the layer down into Toolbox:
a new image is created that contains this layer only.

Finally, you can
put the layer down into another image:
this layer will be added to the layer list, above existing
layers.

2.1.3.
Layer masks

Figure 14.4.
“Add mask” dialog

Overview

A transparency mask can be added to each layer, it's called Layer
mask. A layer mask has the same size and same pixel number as the
layer to which it is attached. Every pixel of the mask can then be
coupled with a pixel at the same location in the layer. The mask is
a set of pixels in gray-tone on a value scale from 0 to 255. The
pixels with a value 0 are black and give a full transparency to the
coupled pixel in the layer. The pixels with a value 255 are white
and give a full opacity to the coupled pixel in the layer.

To create a layer mask start with a right click on the layer to call
the context menu and select Add layer mask
in the menu. If the menu item is grayed first select
Add Alpha channel in the same menu. A dialog
appears where you can initialize the content of the mask:

White (full opacity):
the mask is white
in the Layer Dialog. So, all pixels of the layer are visible
in the image window since painting the mask with white makes
layer pixels fully visible. You will paint with black to make
layer pixels transparent.

Black (full transparency): the mask is
black in the Layer Dialog. So, the layer is fully transparent
since painting the mask with black makes layer pixels
transparent. Painting with white will remove the mask and make
layer pixels visible.

Layer's alpha channel: the mask is
initialized according to the content of layer Alpha
channel. If the layer still contains transparency it's
copied in the mask.

Transfer layer's alpha channel:
Does the same thing as the previous option, except that it
also resets the layer's alpha channel to full opacity.

Selection : the mask is initialized
according to pixel values found in the selection.

Grayscale copy of layer: the mask
is initialized according to pixel values of the layer.

Channel: The layer mask is initialized
with a selection mask you have created before, stored in the
Channel dialog.

Invert mask : This checkbox allows you to
invert : black turns to white and white turns to black.

When the mask is created it appears as a thumbnail right to the
layer thumbnail. By clicking alternatively on the layer and mask
thumbnail you can enable one or other. The active item has a white
border (which is not well visible around a white mask). That's an
important point. Always keep the Layers Dialog prominently when
working with masks, because you can't see, looking at the canvas,
which of the layer or the mask is active.

Pressing Alt (or
Ctrl+Alt and
click on the layer mask thumbnail) is equivalent to the
Show Layer Mask
command : the layer mask border turns to green. If you press
Ctrl the border is red and the result is
equivalent to the
Disable Layer Mask
command. To return to normal view redo last operation. These
options are for greater convenience in your work.

Layer Mask example

Figure 14.5.
A layer with layer mask

This image has a background layer with a flower and another
blue one, fully opaque. A white layer mask has been added to
the blue layer. In the image window, the blue layer remains
visible because a white mask makes layer pixels visible.

Figure 14.6.
Painting the layer mask

The layer mask is active. You paint with black color, which
makes the layer transparent : the underlying layer becomes
visible.